Dexter finds one fingerprint in the car and somehow immediately knows it belongs to the killer, it could have been anybody's. He then doesn't do what he always does and check the guy out and see if he is guilty, hunts him down and kills him.
When Dexter is staging the scene to fake Travis's murder as ritual suicide, the knife changes from Dexter's knife to the ancient sword then back to the knife. Dexter then removes the knife and inserts the sword.
When the forensics team are at the place where Mike was killed, Dexter takes out his iphone and used its touchscreen to scan for fingerprints of the corpse in the car trunk. A touchscreen cannot scan for fingerprints because it doesn't work that way.